The present invention relates to a golf ball material which has a good thermal stability, flowability and processability and from which there can be obtained high-performance golf balls endowed with excellent properties such as rebound and durability. The invention also relates to a golf ball that includes as an essential component a molding made from such a golf ball material, and to a method for preparing such a golf ball material.
Lately, ionomer resins have been widely used in cover materials for golf balls. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic or maleic acid, in which some of the acid groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics in terms of durability, rebound and scuff resistance of the ball.
Today, the base resins used in cover materials for golf balls are mostly ionomer resins, but a variety of modifications are being made to address the constant desire by golfers for golf balls having a high rebound and an excellent flight performance.
For example, to improve the rebound and cost characteristics of ionomer cover materials, U.S. Pat. Nos. 5,312,857, 5,306,760 and International Application WO 98/46671 describe cover materials composed of ionomer resins to which a large amount of metallic soap has been added.
However, the metallic soap in such cover materials undergoes decomposition and vaporization during injection molding, generating a large amount of fatty acid gas. As a result, molding defects tend to arise. Moreover, the gas that is generated adheres to the surface of the molding, markedly lowering its paintability. The rebound performance obtained with these cover materials does not differ to any considerable extent from the rebound obtained with ionomer covers of the same hardness which contain no metallic soap; either the rebound in both cases is about the same or, at most, only a small positive effect is observable from the addition of a metallic soap. Hence, such addition does not markedly increase rebound. Moreover, depending on the type of metallic soap used, the processability and the rebound are sometimes greatly deteriorated or degraded, making the cover material entirely unfit for practical use.
An ionomer for use as a golf ball material has recently been developed in the form of a high rebound resilience material having an interpenetrating network (IPN) structure and a homogeneous phase. The ionomer is obtained by mixing a first component that is typically an ethylene-(meth)acrylic acid copolymer with a second component that is typically a different type of thermoplastic resin so to form a resin composition, then adding a metal ionic species as a third component to neutralize the acid in the first component dispersed within the resin composition (U.S. Patent Application No. 2004/0044136). However, in this prior-art production method, since a metal ionic species such as a metal oxide, metal hydroxide or metal carbonate is used directly in the form of a solid (powder or granular material) and in some case a high acid content in the first component makes it necessary to include a large amount of the metal ionic species required to neutralize the acid, poor dispersion may arise during mixing, leaving some of the metal ions unreacted when the metal ionic species is added. There is a concern that such unreacted metal ions may degrade the physical properties of the ionomer material formed.
In taking into account the viewpoints above, it is an object of the invention to use a low acid-content material having primarily an acid anhydride so as to minimize the amount of metal ionic species required for acid neutralization and enhance the dispersibility and neutralization reactivity, and thereby provide a golf ball material which has a good thermal stability, flowability and processability and from which there can be obtained a high-performance golf ball having excellent properties, such as durability, scuff resistance and optimal hardness, without sacrificing or losing rebound. Another object of the invention is to provide a golf ball containing as an essential component a molding made from such a golf ball material. A further object of the invention is to provide a method for preparing such a golf ball material.